A two-headed shark fetus discovered in Florida Keys waters is not a conjoined twin but a mutant two-headed shark, a study released this week states.

The condition is called axial bifurcation, a deformity that results from the embryo beginning to split into two separate organisms, or twins, but doing so incompletely. The condition is also called dicephaly, or two heads, said Michael Wagner, an author of the report and researcher at Michigan State University.

The fetus was inside a bull shark caught off the Keys in April 2011.

The shark is the subject of a report in the Journal of Fish Biology that was co-authored by Florida Keys Community College Dean of Marine Science and Technology Patrick Rice.

A local fisherman gave Rice the specimen.

The report and accompanying photo have gone viral on the Internet.

Two-headed sharks are rare, with only about six reported cases in history. It's the first case in a bull shark, Wagner said.

"This is a rarely viewed phenomenon," he said.

The shark had two heads, two stomachs and two hearts before merging into one creature, Wagner said.

It was among several extracted from the mother shark.

The other baby sharks were released back into the ocean and swam away, Wagner added.

It was doubtful the two-headed shark would have survived in the wild, as it was about half the size it should have been and had a much reduced spinal column, the researcher said.

Wagner could not pinpoint the cause of the mutation, speculating it was some kind of "natural" genetic mutation. There was no evidence that it was from pollution, radiation or any kind of man-made causes, he said.

If the mutation was due to the 2010 Deepwater Horizon oil spill, other fish in the area would suffer from similar mutations, which is not the case, Wagner said.

The fetus, about a foot long, was kept intact and not dissected when it was studied. The shark was shipped to Michigan State University, where it was X-rayed and underwent an MRI.

It's since been returned to Florida Keys Community College, where it will remain in the science department.